The principal method of manufacturing plastics baths, shower-trays and the like consists of forming a thermoplastic shell with a high surface finish and attaching a second layer by spraying on a mixture of fibreglass and polyester resin. The sprayed-on material is then subjected to the successive stages of rolling, in order to consolidate the material and remove entrapped air, and curing at about 70.degree. F. (21.degree. C.) in order to harden the material. To impart rigidity a base made of chipboard, for example, is adhered to the bottom of the unit. This also serves to receive a support frame.
The disadvantages of this process can be summarised as follows.
1. An externally adhered base board may under certain circumstances become delaminated. It may also lack durability under unfavourable atmospheric conditions, e.g. high humidity; it is susceptible to attack by pests; and it detracts from the aesthetic appeal of the unit to the customer.
2. The existence of a fibre/resin spraying stage necessitates the adoption of stringent precautions to maintain an atmosphere that complies with health and safety regulations.
3. The spraying stage is a manual operation and in order to ensure compliance with regulations regarding the minimum weight for a given area the quantity of material applied will frequently be much in excess of requirements.
4. The cycle time (i.e. the manufacturing time per unit) is fairly lengthy, typically about 25 minutes.
Reaction injection moulding (RIM)--which expression herein includes reinforced reaction injection moulding (RRIM)--is a closed-tool technique in which reactant streams are brought together under pressure to form a polymer directly in the mould. This technique is described by C. M. Hall in an article entitled "Reaction Injection Molding" in Modern Plastics Encyclopedia 1982-1983, McGraw-Hill Inc. (1982), pages 355-357, and has found particular application in the automobile industry because of the demand for lightweight structural parts such as bumper units and fascia panels. The technique and apparatus for carrying it out have been described in the patent literature; for example, U.S. Pat. No. 3,981,671 (B. L. Edwards) discloses a mould suitable for the liquid reaction moulding of solid polyurethane articles and means for clamping the mould components, and U.S. Pat. No. 3,908,862 (R. S. Chandra and K. L. Stone) discloses an apparatus suitable for dispensing predetermined ratios of polyol and polyisocyanate liquids in the reaction injection moulding of polyurethane articles. The teachings of these U.S. patents and of the above-mentioned article by C. M. Hall are incorporated herein by reference.